The present invention relates to sanitary or household papers. It applies for example to toilet paper, paper towels and any paper used for wiping, table napkins, etc.
In general cellulose webs, also called tissue paper, are used in manufacturing these products. This is an absorbent paper of low specific surface weight between 10 and 45 g/m2 made by a wet method from paper fibers. This paper also may include low proportions of chemical additives according to the intended application. It may be made by pressing the still wet web on a heated large diameter cylinder which dries the sheet which is then removed using a metal blade applied to the cylinder transversely to its direction of rotation. The purpose of this procedure is to crepe the web which then exhibits corrugations running transversely to its direction of advance. Creping imparts some elasticity to the sheet while also increasing its thickness and imparting to it properties of hand. Another known mode of manufacture comprises a first and at least partial sheet drying stage using a cross-flow of hot air. The sheet then may be creped or not.
The object of the present invention is marking a sheet consisting of one or more cellulose plies to create a pattern which simulates a watermark.
The expression xe2x80x9cmarkingxe2x80x9d used herein denotes the compression of given parts of one side of the sheet in order to produce thereby compacted zones and to significantly reduce its thickness in the zones without thereby forming corresponding salients on the opposite side of the sheet.
A watermark is a design printed into the thickness or body of paper and is visible when viewed for transparency. A watermark is called xe2x80x9cwet watermarkxe2x80x9d or xe2x80x9ctrue watermarkxe2x80x9d when it is introduced during paper manufacture, in the wet process and before drying. Illustratively a wet watermark is made by printing a pattern present on a rotating cylinder in contact with the still wet sheet.
A xe2x80x9csimulatedxe2x80x9d or xe2x80x9cdry watermarkxe2x80x9d results from an imprint, which looks similar to a xe2x80x9cgenuine watermarkxe2x80x9d, applied to paper after its manufacture. A number of different implementing modes already have been proposed, however, they generally relate to applications other than sanitary or household papers.
In its European Patent B 0 864 014, applicant proposed applying a pattern similar to a watermark to an absorbing cellulose web. This procedure consists in deforming the stressed sheet by making it pass between a hard engraved roll and a mating roll exhibiting a hardness greater than 40 Shore D at a pressure of at least 150 kg/cm2 at a marking rate of at least 50 m/min. Preferably the engraved cylinder is made of steel and it is heated to a temperature between 50xc2x0 C. and 100xc2x0 C. Some modest amount of water may be added.
This procedure allows making a product with a pattern simulating a watermark, the marked surface being smooth, glossy and translucid, the product thickness being small compared to the other dimensions. Applicant has endeavored improving this procedure and in particular to make high contrast watermarks without requiring heating means.
This object is attained by the method of the invention.
The method of the invention for marking a watermark-simulating pattern onto a sheet composed of at least one ply of cellulose web and consisting in deforming by stressing the sheet between a first rigid roll having a smooth surface and at least one second rigid roll fitted with salient elements is characterized in that a film of additive enhancing watermark formation is added in an unheated manner to at least one of the sheet""s sides during marking.
Preferably, the additive is based on mineral oil. The additive is applied to the surface and enters the sheet""s thickness while the sheet is being deformed. It is applied in an unheated manner, that is without application of external heat, at ambient temperature. In this manner, optimal results are attained using simple means.
Preferably the additive is applied to the first roll, to the salient elements of the second roll, or to both rolls. In the latter case, the decoration is very visible both on the marked side and on the opposite side.
Different ways of applying the additive may be considered: coating, spraying, atomizing in particular. As regards spraying, the additive may be applied directly on the roll(s) during marking. However, in an advantageous implementation of the invention, the additive may be applied by coating. Coating may be carried out using apparatus fitted with a screened cylinder. In the description below, the additive is applied by contact between the surface(s) to be coated, in this instance the roll(s), and an absorbent means is impregnated with the additive. The additive is then transferred from the surface(s) of the roll(s) to the surface(s) of the sheet during the marking operation.
A small quantity of additive is enough for application so that the finished product contains less than 1% by weight, preferably between 0.2 and 1% and, in particular, between 0.2 and 0.5% by weight of additive relative to the weight of the sheet.
Preferably the additive is a white mineral oil used for medical or nutritional purposes. Its dynamic viscosity at ambient temperature (about 25xc2x0 C.) is between 15 and 80, preferably between 20 and 30 mPa.s as defined in standard NFT60-100.
This method allows creating varied patterns. In particular the salient elements of the second roll constitute pattern units subtending an area between 20 and 50 mm2, preferably between 30 and 40 mm2. In particular salient elements comprise continuous segments of linear form. Preferably the ratio of the marked surface to the total sheet surface is less than 20%.
Moreover, a force is exerted on the second roll and corresponds to a linear pressure on the sheet of approximately between 20 and 100 kg/cml, preferably between 40 and 90 kg/cml.
In one advantageous implementation of marking to make a sheet of which the (machine) width in particular is between 250 and 3200 mm, the sheet is deformed between a first roll which is the width of the sheet and at least one second roll of a width between 15 and 90 mm.
In particular, a plurality of second rolls are arrayed along the length of the first roll. Each roll may be an assembly of several disks.
The manufactured sheet comprising at least one ply of cellulose web of a specific surface weight between 10 and 45 g/m2 and exhibiting a marked zone simulating a watermark is characterized in that it is impregnated with an additive, in particular a mineral oil additive. Advantageously the proportion of additive is less than 1% by wt. relative to the weight of the sheet.